Renal clearance of cephaloridine was lower than inulin clearance at low plasma concentrations of cephaloridine and higher than inulin clearance at high plasma levels. Since changes in protein binding of the drug cannot account for the effects, the data suggest that cephaloridine is reabsorbed by the kidney in the rat, in addition to being filtered and secreted. In the anesthetized rat, cephaloridine increased significantly the rate of endogenous urate urinary excretion and the renal clearance of endogenous urate. The plasma concentration of endogenous urate was decreased. This uricosuric effect of cephaloridine suggests that uric acid and cephaloridine are reabsorbed by the same transport system. Experiments are now in progress to measure the effects of cephaloridine on the uptake of urate by rabbit renal cortical slices.